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Advances in CT Scanner X-Ray Tubes

















Wilhelm Roentgen, is best known is best known for the discovery of "Roentgen Rays", now univerally known as x-rays. Around this time, in 1985, various scientists were investigating the movement of electrons through a glass apparatus known as a Crookes tube. Roentgen wanted to visually capture the action of the electrons, so he wrapped his Crookes tube in black photographic paper. When he ran his experiment, he noticed that a plate coated with a fluorescent material, which just happened to be lying nearby the tube, glowed. This was unexpected, because no visible light was being emitted from the wrapped tube. Upon further investigation, he found that indeed there was some kind of invisible light produced by this tube, and it could penetrate materials such as wood, aluminum, and even human skin.

As the field of radiography expanded, x-ray technology steadily improved. One of the major limitations of conventional x-rays was that they lacked depth; therefore many internal structures were superimposed on each other, making it difficult to read results. With the help of computers, scientists developed methods to solve this problem. One such method was computed tomography (CT), or computerized axial tomography (CAT). The first CT scanner was demonstrated in 1970 by Godfrey Hounsfield and Allen Cormack. Over the next 20 years, significant advances were made in CT scanner design, which have resulted in the high-quality imaging scanners used today, and still constantly improving.

A CT scanner x-ray tube is a special type of vacuum-sealed, electrical diode that was designed and developed to produce x-rays. The CT scanner x-ray tube is comprised of two electrodes: the cathode and anode. To generate x-rays, a filament in the cathode is charged with electricity from a high -voltage generator. This causes the filament to heat up and emit electrons. Using their natural attraction and a special focusing cup, the electrons travel directly toward the positively charged anode. X-rays are indiscriminately released when the electrons strike the anode. The anode, which can be rotating or not, then conducts the electricity back to the high-voltage generator to complete the circuit. To focus the x-rays into a beam, the CT scanner x-ray tube is contained inside a protective housing. This housing is lined with lead, except for a small window at the bottom. Functional x-rays are able to escape out this window, while the lead prevents the escape of stray radiation in other directions.

CT scanner x-ray tubes have gone through several generations of technological evolution. In the third generation, developers realized that if a pure rotational scanning motion could be used, rather than the slam-bang translational motion, then it would be possible to use higher power, rotating CT scanner x-ray tubes and therefore improve scan speeds in thicker, harder to penetrate body parts. A standard machine which most x-ray technicians are familiar with, uses a large fan beam, so that the patient is completely covered by the fan and the detector elements are aligned along the arc of a circle centered on the focus of the CT scanner x-ray tube. The CT scanner x-ray tube and detector array rotate as one through 360 degrees, different projections are attained during rotation by pulsing the x-ray source, and bow-tie shaped filters are chosen to suit the body or head shape by some manufacturers to avoid extreme variations in signal strength.

The fourth generation of CT scanners uses rotate-fixed ring geometry, where a ring of fixed detectors completely surrounds the patient. The x-ray tube rotates inside the detector ring through a full 360 degrees with a wide fan beam producing a single image.

A limiting factor in image acquisition used to be the CT scanner x-ray tube. The need for long, high intensity exposures and very stable output placed enormous demands on both the CT scanner x-ray tube and generator (power supply). Very high performance rotating anode CT scanner x-ray tubes were developed to keep up with demand for faster imaging, as were the regulated 150 kV switched mode power supplies to drive them. Current CT scanning systems have power ratings up to 100 kW.

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